Seams and slots in equipment enclosures can result in emissions leakage
that affect radiated emissions compliance. Often EMC personnel will use magnetic
loops to find such leakage. This Technical Tidbit covers an alternate way
to investigate radiation from seams and slots using a voltage measurement
that can be roughly correlated to the emissions.

Figure 1 shows a high bandwidth (1.8 GHz) differential probe(~400K file) being used
to measure the voltage across a seam in a chassis. With proper interpretation,
this measurement can give an estimate of the emissions potential of this
seam.
If the dimensions of the seam and the surrounding metal are a substantial
fraction of a wavelength at a frequency of interest, then one could
consider the metal as an antenna to be driven by the voltage across the seam.

A dipole that is tuned to 1/2 wavelength at the frequency of interest,
has a low driving point input impedance (~70 Ohms). Under this condition,
it takes only about 15 microamps of current to causes a a potential Class
A (industrial) emissions compliance problem. The voltage input to the dipole
necessary to drive a current of 15 microamps into 70 Ohms is only about 1
mV.

The metal may have a much broader spectrum over which it can radiate than a dipole. However, if we use the same numbers
as for a dipole (we are only looking for an estimate), then a voltage across
the seam on the order of a few millivolts (due to currents or fields inside
of the enclosure) might be a problem for emissions. It is current
that results in radiation in the far field where emissions are measured. The question
is whether a voltage across the seam will result in a current that will, in turn, flow on
the outside surface of the metal enclosure that is sufficient to result in emission problems.

There are a few points to consider.

Is the measured voltage due to a source internal to the equipment?
Turn off the equipment to find out. I call this a null experiment.

Is
the measured voltage due to voltage across the seam, as opposed to probe common
mode response? Short the probe tips together and then to each side of the
seam, one side at a time. This is another null experiment. The result should
be much lower than the measured voltage across the slit. The result of this
null experiment (shorting the probe tips) will also indicate if a magnetic
field in the area is inducing a voltage into the probe tip loop in accordance
with the descriptions of Faraday's law. In either case, if the shorted probe
response is similar to the measured voltage across the slot (or seam), it
may not be possible to make this voltage measurement.

If the measured voltage is really across the seam,
the final question to answer is: Does that voltage result in current flow?
Without current flow, there would not likely be significant emission. For
an enclosure whose metal is a substantial fraction of a wavelength the answer
is probably yes, there will be current flow and emission. One way to check
is to put a 75 Ohm resistor across the seam. If the impedance across the
seam is low, the voltage will not change appreciably, perhaps only a few
dB. If the impedance is high, the addition of the resistor will change the
result substantially. This could happen if standing waves make the seam a
high impedance node. A low impedance across the seam can be due to the metal
acting as a low impedance antenna, thus permitting RF current to flow. This
is not the only possibility, but a low impedance across the seam coupled
with a few millivolts of voltage across the seam probably means that there will be significant emission potential.

One caution: To get the best results from the measurement discussed
here, the differential probe should have an input impedance of at least a
few hundred Ohms at the frequency of interest. Many active differential probes
have an input impedance too low, as low as 20 Ohms when used with small probe
tips, for a good measurement. See the Technical Tidbit: August 2002, Probe Input Impedance Revisited
- Active Probes for more details.

Figure 2 shows an oscilloscope measurement across a slot (a spectrum
analyzer was not available) on an operating piece of equipment. Given the
scope scale of 10 mV (full scale), any signal that can be displayed would
represent a possible compliance problem. There is a strong frequency component
of about 500 MHz having a peak value of about 4 mV. There is also a smaller
component at about 4 GHz. Given the scope frequency response was on the order
of 1.5 GHz, the 4 GHz component is really much larger than shown and did,
in fact, cause an emissions problem. Filling the slot with an EMI gasket
reduced the scope reading to "flat line," indicating that the emissions due
to the slot would be significantly reduced.

Although not a substitute for an emissions test, a measurement as
described may increase the confidence factor of passing an emissions test
while the equipment is still in the development lab. Lab bench tests,
such as this one, can lower development costs and speed equipment to final
approval.